1. Field of the Invention
The present invention relates to LED illuminating method and apparatus for an image measuring device. In particular, the present invention relates to LED illuminating method and apparatus for an image measuring device, which are suitable for use in an image measuring device that uses a common high-brightness LED, and performs low-brightness continuous illumination and high-intensity stroboscopic illumination or high-brightness continuous illumination, on a measuring object (hereafter referred to as “work piece”).
2. Description of Related Art
Conventionally, as shown in FIG. 1, a CNC image measuring device performs a measurement of one measuring position, by moving a measuring stage with respect to an imaging device, such as a CCD camera, etc., stopping the measuring stage at the measuring position, acquiring image information of a work piece by adjusting an amount of light of illumination, and setting a measuring tool to and performing image processing, such as an edge detection, on the acquired image information. The measurement is repeatedly performed at all measuring positions, including measurement 1, measurement 2, and so on, to perform measurement of necessary positions. Hereinafter, this measuring mode is referred to as a “standard measuring mode.”
On the other hand, in order to improve a measuring throughput, an image measuring device is known, which is provided with a measuring mode in which a measuring operation is performed without stopping the measuring stage with respect to the imaging device at the measuring position. Hereinafter, this measuring mode is referred to as a “non-stop measuring mode.” See, for example, Japanese Translation of PCT International Application No. 2004-535587 (paragraphs [0005]-[0006], FIG. 2). As shown in FIG. 2, this image measuring device performs image measuring by inputting instantaneous image information, which is acquired by stroboscopically illuminating the work piece or imaging the work piece with a CCD camera having a shutter, without stopping the measuring stage at measuring positions. The non-stop measuring mode has an advantage that a high-speed measuring is realized without substantially degrading measurement accuracy, by properly setting the relationship between the moving speed of the stage and the width of the stroboscopic pulse.
In recent years, an LED having a higher brightness, i.e. a high-brightness LED, is developed and has become commercially available. Accordingly, the continuous illumination for the standard measuring mode and the stroboscopic illumination for the non-stop measuring mode may be realized with the high-brightness LED.
However, when an LED is continuously lit, e.g., when a work piece is continuously observed in the standard measuring mode, the high-brightness LED, lit with the rated power, is too bright. Therefore, the LED must be driven with a small power. However, the high-brightness LED has characteristics that it is stable in high brightness lighting with a large current, but in low brightness lighting with a small current, a constant brightness cannot be maintained, the brightness varies with time, and thus it is unstable. Accordingly, in order to realize stable low brightness lighting of the high-brightness LED, the power supplied to the LED is reduced, generally by supplying a large current during a LED lighting period, but performing intermittent lighting (pulse lighting).
On the other hand, in the same image measuring device, when an image is acquired while the work piece is moving in the non-stop measuring mode, a method is known in which image data is acquired by performing instantaneous illumination (referred to as stroboscopic light emission) on the work piece by high-brightness light, to prevent image blurring.
In this case, in order to acquire an unblurred image, light emission of the LED requires stroboscopic light having a short lighting interval. In order to acquire an image with a high accuracy, the stroboscopic light must be controlled, with a high degree of accuracy, in such a manner that constant lighting intervals, a constant light amount (brightness or light emission period), and so on, are maintained.
However, when an LED is lit intermittently by giving a command of stroboscopic illumination as shown in FIG. 3B to a light source of pulse lighting as shown in FIG. 3A, it is difficult to reproduce a stable lighting period, due to the difference in cycle between the lighting interval of stroboscopic light emission and the intermittent lighting of pulsed light emission, as shown in FIG. 3C.
A low-brightness LED and a high-brightness LED may be used respectively for continuous illumination and stroboscopic illumination. However, this is not desirable, since the number of optical components increases.
On the other hand, Japanese Patent Application Publication No. 2006-135297 discloses an LED that switches driving current. In Japanese Patent Application Publication No. 2006-135297, when a target value of the amount of light emission is equal to or lower than a predetermined value, a PWM control is performed in which a driving current value is maintained constant at the best current value, and ON and OFF of the driving current is controlled. On the other hand, when the target value of the amount of light emission is higher than the predetermined value, a current value control is performed that changes the continuously supplied driving current value.
However, Japanese Patent Application Publication No. 2006-135297 only discloses a technology that drives an LED with improved luminance efficiency, and does not relate to a technology that switches between continuous illumination and stroboscopic illumination, as disclosed in the present application.